A thermostatic mixing valve such as described in U.S. Pat. No. 6,648,234 has a housing and a stem extending along an axis in the housing and axially displaceable to adjust the thermostatically regulated temperature of the valve. A tubular core body fixed in the housing and aligned with the stem holds a coupling body axially displaceable in the core body, having an inner end axially engageable with the stem and an outer end formed with a screwthread. Interengaging formations on the coupling body and the core body inhibit rotation of the coupling body in the core body while permitting axial displacement of the coupling body in the core body. A knob rotatable about the axis has a screwthread meshing with the screwthread of the coupling-body outer end. A retainer engaged between the knob and the core body inhibits axial displacement of the knob on the core body and permits rotation of the knob on the core body so that rotation of the knob on the core body axially shifts the coupling body in the core body.
Thus with this arrangement the control knob can rotate but not move axially, but its rotation is converted into the required axial displacement of the coupling body and valve stem. Thus the user rotates the knob to adjust temperature but the axial position of the knob remains the same.
The problem with such an arrangement is that the various elements of the knob assembly, which are typically made of plastic, are occasionally subjected to substantial forces radial of the rotation axis. They can be damaged for instance if a user pulls the knob too forcibly to the side or something is dropped on the knob.